Combustion processes in which fuels containing sulphur, nitrogen and chlorine are burnt basically release sulphur dioxide, nitrogen oxides and hydrochloric acid as well as halogenated organic compounds. Due to their toxicity, these substances are often subject to national emission limits which have been laid down, for example, for the Federal Republic of Germany by the legislator in the 17th Federal Emissions Control Ordinance (17th BImSchV) for compounds from waste incineration plants.
During the combustion in incineration plants of nitrogen-containing fuels, such as domestic refuse or various biomasses, such as stalk plants or crops, nitrogen oxides (NOx) are substantially produced from the nitrogen (N) bound in the fuel.
In industrial firings, solid fuels are burnt in two stages. The solid fuel is burnt in a first stage by adding primary air. Primary air is generally added in a sub-stoichiometric manner. The incomplete burn-out resulting therefrom, which is to be attributed to a local lack of oxygen in the combustion bed, of the primarily formed smoke gases means that secondary air has to be added and mixed into the waste gas which is still of high calorific value, as a result of which an afterburning operation is initiated. In the process, very high temperature peaks result locally, NO or N2 being ultimately formed by complex reactions from the ammonia (NH3) and hydrogen cyanide (HCN) formed primarily from the fuel nitrogen during the waste gas burn-out. However, a thermal formation of nitrogen oxide (NOx formation) from atmospheric nitrogen is comparatively low due to the relatively low temperature level in these combustion plants.
In the so-called SNCR (selective non-catalytic reduction) process, ammonia (NH3) or other N-containing reducing agents, such as aqueous ammonia solutions or urea, are sprayed into the oxygen-containing waste gas downstream of the waste gas burn-out zone within a temperature range of 850 to 1050° C. for the selective non-catalytic reduction of nitrogen oxides in the waste gas. Excessive temperatures result in the formation of NO (nitrogen monoxide), while temperatures which are too low increase the NH3 slip. An aqua ammonia (NH4OH) is generally used. During evaporation of this solution, ammonia (NH3) is released and NO is reduced into N2 at the previously mentioned temperature level.4NH3+4NO+O2→4N2+6H2O  (1)
NH3 can also be used as a neutralising agent in SO2 scrubbers which are operated at neutral or slightly acidic pH. See P. Strasser, V. Fütterer: Betriebserfahrungen mit der Rauchgasentschwefelungsanlage des Rheinhafen-Dampfkraftwerkes und des Groβkraftwerkes Mannheim; User report Abgasreinigung, special edition Staub Reinhaltung der Luft (1987) Book 10, pages 63-68. If an acid-operated first scrubber which generally operates at pH values <1 and in which HCl is separated practically quantitatively is connected upstream of the SO2 scrubber, then SO2 is selectively separated in the second washing stage connected downstream (SO2 scrubber) using NH3 at pH≦7. See DE 197 31 062 C2.2NH3+H2O+SO2→(NH4)2SO3  (2)
The ammonium sulphite which has formed ((NH4)2SO3) is oxidised into ammonium sulphate ((NH4)2SO4) with the oxygen contained in the waste gas or by additionally supplied oxidising air.(NH4)2SO3+½O2→(NH4)2SO4  (3)
Halogenated organic compounds are, for example, polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) which are formed in combustion processes, such as in refuse incineration, and are discharged with the waste gas. The formation of PCDD/F is basically caused when the carbon-containing and chloride-containing fly ash deposits on the boiler surfaces or during the dedusting operation in a temperature range of >200° C. The formation maximum of PCDD/F is within a temperature range of from approximately 300 to 350° C.
Due to their toxicity, the legislator in the Federal Republic of Germany has laid down a limit for the emission of these compounds from waste incineration plants of 0.1 ng TEQ/Nm3 (TEQ=toxicity equivalent) in the 17th Federal Emissions Control Regulation (17th BImSchV). Based on today's knowledge, this limit for PCDD/F in the combustion waste gas is unsustainable by merely optimising the firing conditions. In this respect, it is known in the art to reduce the concentration of PCDD/F in the combustion waste gas below the prescribed limiting value with an additional smoke gas cleaning operation connected downstream of the combustion. Waste incineration plants substantially consist of a combustion chamber optionally with a boiler, at least one dust collector connected downstream and wet scrubbers as well as additional adsorptively and/or catalytically acting smoke gas cleaning processes for NOx and/or PCDD/F reduction. The SNCR process is also frequently used to reduce NOx.
R. D. Griffin, “A new theory of dioxin formation in municipal solid waste combustion”, Chemosphere, Vol. 15, Nos. 9-12, pp. 1987-1990, 1986 and K. Raghunathan, B. K. Gullett: Role of Sulfur in Reducing PCDD and PCDF Formation; Environ. Sci. Technol. 30 (1996) pages 1827-1834 generally describe that it is also possible to considerably reduce the PCDD and PCDF content in a waste gas merely by means of a sulphur excess relative to the chlorine stock during a combustion procedure. In this respect, the ratio of sulphur dioxide to hydrochloric acid in the smoke gas formed during combustion is of particular significance, resulting by sulphation reactions, in a considerable reduction of the chloride contents of the fly ash with an increasing ratio of sulphur dioxide to hydrochloric acid.2(Na, K) Cl+SO2+½O2+H2O→(Na, K)2SO4+HCl  (4)
Metal chlorides exhibit an analogous behaviour to that of alkalis. During sulphation, the chlorides of the fly ash are converted into sulphates. The resulting low-chloride fly ash gives rise to a drastic reduction in the PCDD and PCDF formation potential and thus leads to a significant reduction in the PCDD/F concentration in the waste gas. Low-chloride fly ash deposits also entail a reduction in the boiler corrosion rates.
DE 103 38 752 B4 describes a process in which SO2 is selectively separated from the smoke gas in at least one scrubber and is recycled into the combustion chamber as SO2 or as sulphuric acid. Due to the selective separation of SO2 in a scrubber, highly concentrated SO2 is available for recycling. A result of recycling is the concentration of SO2 and thus the reduction of the Cl/S ratio in the waste gas in the process zones relevant to the PCDD/F formation. However, this process does not allow a reduction in NOx. In the process, an aqueous mixed salt solution of chlorides and sulphates is formed as a residue